1. Field of the Invention
The present invention relates to inks, particularly, inks used in printing of an ink-jet system, and ink-jet printing processes. The present invention also relates to ink containers, ink sets and ink-jet printing apparatus. The present invention is applicable to all apparatus by which printing can be performed on various kinds of printing media such as paper, cloth, leather, nonwoven fabric and OHP paper, for example, business machines and instruments such as printers, copying machines and facsimiles, to which an ink-jet system is applied.
2. Related Background Art
An ink-jet system is a system having various merits of low noise, low running cost, high-speed printing, easy miniaturization of apparatus, easy formation of color images, etc. and hence widely used in printers, copying machines and the like. In such a printer or the like, inks used are generally selected from the viewpoints of ejection properties, printing properties such as fixing ability, print quality such as feathering or bleeding of printed images, optical reflection density and coloring ability, etc.
It is widely known that inks are roughly divided into two kinds of inks of dye inks and pigment inks according to a coloring material contained therein. Of these, the pigment inks have merits of, for example, being excellent in water fastness and light fastness compared with the dye inks, and permitting the achievement of clear character quality. A pigment contained in a pigment ink is generally stably dispersed in the ink using a dispersing agent such as a polymeric dispersant. A specific action thereof is as follows. Namely, pigment particles are adsorbed on the polymeric dispersant to conquer intermolecular force acting between the pigment particles, which brings on aggregation of the pigment particles, making good use of mainly electrical repulsion force and the like of the polymeric dispersant, thereby stably dispersing the pigment particles in the ink. Accordingly, it is necessary to add the polymeric dispersant to the ink according to the amount of the pigment. When printing is conducted on plain paper with such an ink by means of an ink-jet system, pigment particles aggregate due to the penetration of a solvent such as water in the ink into the paper and evaporation thereof into the air. At this time, as behavior on the paper, the cohesion of the pigment particles becomes stronger as the amount of the polymeric dispersant increases. Therefore, the diameter of an ink dot formed on the printing medium by the ink having a certain volume ejected from an ink-jet head becomes small, and as the shape thereof, a distorted shape formed upon impact on the paper almost remains as it is. It is hence necessary to control the volume of the ink ejected from the ink-jet head larger in order to provide ink dots having an optical density sufficient to form an image and a dot diameter required for printing free of occurrence of white stripes and the like. However, this requirement may have incurred the slowdown of fixing of the ink to the printing medium or deteriorated the rub-off resistance of the printed image in cooperation with the lowering of penetrability of the ink due to strong cohesion of the pigment particles on which the polymeric dispersant has been adsorbed.
It is also considered that a penetrant is contained in an ink in order to improve the penetrability of the ink into printing media so as to enlarge the diameter of an ink dot and enhance the fixing ability of the ink. However, in some cases, this method may cause phenomena unfavorable for intending the formation of a high-quality printed image, such as deterioration of dot shape (deterioration of peripheral shape of dot such as so-called feathering) and penetration of the ink into the back surface of the paper (so-called strike-through). Besides, since a coloring material penetrates into the interior of the printing medium, the optical density (OD) of ink dots may not become very high in many cases even when the dot diameter is relatively enlarged.
Further, inks using a self-dispersing pigment have been proposed. According to these inks, the diameter of a dot can be enlarged probably because the cohesion of the pigment on paper is weak compared with the inks containing a pigment dispersed by the above-described dispersing agent. However, it is not yet sufficient.
As described above, inks and printing processes, which satisfy various factors controlling the quality of a printed image, for example, the fixing ability of the inks, enlargement of ink dot diameter, evenness of density within an ink dot, high optical density of ink dot itself, etc., at a high level, and also stability as inks, particularly ejection stability as ink-jet inks, are yet in a fair way to research aimed at still more improvement.
It is an object of the present invention to provide an ink that the ink droplet applied to the printing medium moderately spreads in the lateral directions of a printing medium and is prevented from penetrating in the thickness direction of the printing medium, and consequently can provide a dot that is high and even in optical density of image within the dot and has an excellent external shape in that feathering is scarcely observed.
Another object of the present invention is to provide an ink-jet printing process which can form a high-quality image having a high optical density of image while reducing the amount of an ink applied to a printing medium.
A further object of the present invention is to provide an ink-jet printing apparatus capable of stably printing a high-quality image, and an ink container and an ink set used in this process.
A still further object of the present invention is to provide an ink for forming images and an image forming process, which can provide an optical density of image and image quality that can satisfy properties finally required of an ink forming a dot or an ink forming a pixel (including an overlapped dot portion by a dot matrix such as 3xc3x973 or 4xc3x974).
The above objects can be achieved by the present invention described below.
In an aspect of the present invention, there is thus provided an ink comprising a first pigment, a second pigment and a dispersant, both of the pigment being dispersed in an aqueous medium, wherein the first pigment is a self-dispersing pigment having an anionic group or a cationic group, the group being bonded directly or through an atomic group to a surface of the pigment, the second pigment is a pigment dispersible in an aqueous medium with the dispersant, and the dispersant is an ionic polymeric dispersant having a same polarity as that of the group bonded to the surface of the pigment or a nonionic polymeric dispersant.
In another aspect of the present invention, there is provided an ink used in an image printing process comprising the steps of ejecting an ink from an orifice by means of an ink-jet system; applying the ink to a printing medium; and forming an element of an image, the ink comprising a first pigment, a second pigment and a dispersant, both of the pigment being dispersed in an aqueous medium, wherein the first pigment is a self-dispersing pigment having an anionic group or a cationic group, the group being bonded directly or through an atomic group to a surface of the pigment, the second pigment is a pigment dispersible in an aqueous medium with the dispersant, and the dispersant is an ionic polymeric dispersant having a same polarity as that of the group bonded to the surface of the pigment or a nonionic polymeric dispersant.
In an aspect of the present invention, there is also provided an ink-jet printing process comprising the steps of ejecting an ink toward a printing medium by means of an ink-jet system; and forming an image on the printing medium, wherein the ink comprises a first pigment, a second pigment and a dispersant, both of the pigment being dispersed in an aqueous medium, wherein the first pigment is a self-dispersing pigment having an anionic group or a cationic group, the group being bonded directly or through an atomic group to a surface of the pigment, the second pigment is a pigment dispersible in an aqueous medium with the dispersant, and the dispersant is an ionic polymeric dispersant having a same polarity as that of the group bonded to the surface of the pigment or a nonionic polymeric dispersant.
In an aspect of the present invention, there is still also provided on an ink-jet printing process comprising the steps of ejecting an ink toward an outer surface of a coating layer of a printing medium provided with the coating layer by means of an ink-jet system; and forming an image on the outer surface of the coating layer, wherein the ink comprises a first pigment, a second pigment and a dispersant, both of the pigment being dispersed in an aqueous medium, wherein the first pigment is a self-dispersing pigment having an anionic group or a cationic group, the group being bonded directly or through an atomic group to a surface of the pigment, the second pigment is a pigment dispersible in an aqueous medium with the dispersant, and the dispersant is an ionic polymeric dispersant having a same polarity as that of the group bonded to the surface of the pigment or a nonionic polymeric dispersant.
In an aspect of the present invention, there is further provided an image forming process comprising the steps of:
(i) providing a first ink containing an aqueous medium and a first pigment which is a self-dispersing pigment dispersible in the aqueous medium by itself, the pigment having an anionic group or a cationic group, and the group being bonded directly or through an atomic group to a surface of the pigment;
(ii) providing a second ink containing an aqueous medium and a second pigment dispersible in the aqueous medium with a dispersant, the dispersant being at least one of a nonionic polymeric dispersant and an ionic polymeric dispersant having a same polarity as that of the group bonded to the pigment in the first ink;
(iii) applying the first ink and the second ink on a printing medium respectively so that the first ink and the second ink come into contact with each other in liquid state on a surface of the printing medium; and
(iv) forming an image on the recording medium.
In an aspect of the present invention, there is still further provided an image forming process comprising the steps of:
(i) providing a first ink containing an aqueous medium and a first pigment which is a self-dispersing pigment dispersible in the aqueous medium by itself, the pigment having an anionic group or a cationic group, and the group being bonded directly or through an atomic group to a surface of the pigment;
(ii) providing a second ink containing an aqueous medium and a second pigment dispersible in the aqueous medium with a dispersant, the dispersant being at least one of a nonionic polymeric dispersant and an ionic polymeric dispersant having a same polarity as that of the group bonded to the pigment in the first ink;
(iii) providing a third ink containing a dye having a same polarity as that of the group bonded to the pigment in the first ink;
(iv) applying the first, second and third inks to a printing medium so that the inks come into contact with each other in liquid state on a surface of the printing medium.
As described above, the present invention is based on the finding in the course of the research aimed at the formation of still higher-quality images by ink-jet printing that an ink containing the self-dispersing pigment, the pigment capable of being dispersed in an aqueous medium by a polymeric dispersant and the polymeric dispersant in combination satisfies various factors required for the formation of high-quality images at a very high level and is also excellent in the stability of the ink itself.
More specifically, as the form of the ink until it is ejected from a printing head, the self-dispersing pigment (first pigment) probably functioned as a dispersing agent for the pigment (second pigment) capable of being dispersed in an aqueous medium by a polymeric dispersant, so that the second pigment in the ink was able to stably keep its dispersed state even when the polymeric dispersant in the ink decreased as a whole. On the other hand, when printing was conducted on paper using this ink, the dot diameter thereof was large compared with a dot diameter by an ink comprising the second pigment and the polymeric dispersant for dispersing it or ink comprising the first pigment (self-dispersing pigment) alone. In addition, the dot uniformly diffused on the surface of the paper, was high in optical density and fixed relatively fast.
The reason why such a phenomenon is observed is not clearly known. However, it is considered to be due to the following mechanism. Namely, the second pigment on which the polymeric dispersant has been adsorbed, and the first pigment electrically repulse each other in the ink, and so the cohesion of the pigments becomes at least weak compared with the ink comprising only the pigment dispersed by the polymeric dispersant. When such an ink is applied to the surface of paper, the coloring material in the ink is hard to penetrate in the thickness direction of the paper, since the polymeric dispersant is adsorbed on the second pigment. On the other hand, with respect to diffusion in the lateral directions of the paper surface, it is considered that in the case of the ink comprising the second pigment and the polymeric dispersant, polymer molecules are entangled each other an water is an decreased by penetration of a solvent in the ink into the paper and evaporation thereof, or the polymer molecules crosslink pigment particles, whereby the pigment strongly aggregates, whereas in the ink according to this aspect of the present invention, the entanglement of the polymer molecules or the crosslinking is prevented or suppressed by the coexistence of the first pigment, and strong intermolecular force between the pigments in the ink is relieved by the repulsion between the first pigment and the polymeric dispersant, so that the ink is easy to diffuse in the lateral directions of the paper surface, and the diffusion does not become random because it is affected by the cohesion among the pigment particles though it is relieved. This is considered to appear as insurance of a greater dot diameter and roundness of a dot even by an ejected ink quantity reduced and as good compatibility when plural dots run in a line, i.e., good smoothness. This phenomenon on the paper surface becomes particularly marked when the Ka value of the ink in the Bristow method is lower than 1 mlxc2x7mxe2x88x922xc2x7msecxe2x88x92xc2xd, namely, the penetrability is designed relatively low to a printing medium, and acts favorably on the formation of a high-quality image.
As described above, the inks according to this aspect are good in the dispersion stability in an ink tank, and even in the printing properties, have such great effects that the dot diameter is great, OD is high, rub-off resistance is good, and the roundness of dot is good.
The inks according to this aspect are good even when they are ejected from an ink-jet printing head. Namely, high-speed driving becomes feasible because the viscosity is low, and refilling property to a nozzle is good. With respect to ejection by an ejection system making good use of bubbling by film boiling, ejection velocity and ejected ink quantity become relatively large due to their moderate wettability to the surface of an ejection heater and/or low viscosity resistance against quick bubbling, and fluctuation is also little. Namely, ejection efficiency and ejection stability are good.
Even when an experiment is conducted with the ejected ink quantity controlled so as to become even for various kinds of inks in view of this enhancement of ejection efficiency, the dot diameter of the inks according to this aspect becomes sufficiently great. Accordingly, the enlargement of the ink dot diameter by the inks according to this aspect cannot be explained by the simple reason that the ejection efficiency is enhanced. When printing is conducted by means of the conventional BJ printing head, the use of the inks according to this aspect permits the formation of ink dots having a greater diameter because the ejected ink quality, ejection velocity and spreadability of the inks themselves act as a synergistic effect. The fact that ink dots having a sufficiently great diameter are provided even when an ejected ink quantity is reduced means that the ink thinly spreads in a wide range on the surface of a printing medium. Therefore, the ink is dried quickly, so that the time required for the fixing of the ink can be shortened.
The cohesion of the pigments on the paper surface can be further relieved by adding, to an ink according to this aspect comprising the first pigment, second pigment and polymeric dispersant, a dye having the same polarity as the group bonded to the surface of the first pigment. This contributes to further enlargement of an area factor and moreover has an additional preferable effect that a printed image on a printing medium having a resin coating layer is effectively prevented from causing xe2x80x9ccrackingxe2x80x9d.
In an aspect of the present invention, there is yet still further provided an ink container containing an ink therein, wherein the ink comprises a first pigment, a second pigment and a dispersant, both of the pigment being dispersed in an aqueous medium, wherein the first pigment is a self-dispersing pigment having an anionic group or a cationic group, the group being bonded directly or through an atomic group to a surface of the pigment, the second pigment is a pigment dispersible in an aqueous medium with the dispersant, and the dispersant is an ionic polymeric dispersant having a same polarity as that of the group bonded to the surface of the pigment or a nonionic polymeric dispersant.
In an aspect of the present invention, there is yet still further provided an ink set comprising, in combination, a black ink which comprises a first pigment, a second pigment and a dispersant, both of the pigment being dispersed in an aqueous medium, wherein the first pigment is a self-dispersing carbon black having an anionic group or a cationic group, the group being bonded directly or through an atomic group to a surface of the pigment, the second pigment is a pigment dispersible in an aqueous medium with the dispersant, and the dispersant is an ionic polymeric dispersant having a same polarity as that of the group bonded to the surface of the pigment or a nonionic polymeric dispersant, and at least one ink selected from the group consisting of a yellow ink, a magenta ink and a cyan ink.
In an aspect of the present invention, there is yet still further provided an ink-jet printing apparatus comprising ink container portions which separately contain a black ink comprising a first pigment, a second pigment and a dispersant, both of the pigment being dispersed in an aqueous medium, wherein the first pigment is a self-dispersing carbon black having an anionic group or a cationic group, the group being bonded directly or through an atomic group to a surface of the pigment, the second pigment is a pigment dispersible in an aqueous medium with the dispersant, and the dispersant is an ionic polymeric dispersant having a same polarity as that of the group bonded to the surface of the pigment or a nonionic polymeric dispersant, a yellow ink, a magenta ink and a cyan ink therein, and means for respectively ejecting the black, yellow, magenta and cyan inks by an ink-jet system.
Incidentally, as the yellow, magenta and cyan inks as referred to herein, may be used publicly known inks and inks suitably improved thereon. Inks having a Ka value in the Bristow method of at least 1 mlxc2x7mxe2x88x922xc2x7msecxe2x88x92xc2xd are particularly preferred. According to these ink container, ink set and ink-jet printing apparatus, far excellent images compared with the conventional images can be printed for the above-described reasons.